A DC-DC converter of, for example, a step down type is used in information devices. A switching power supply using a switch is employed in the DC-DC converter. A back type switching power supply is used in a case where grounds of input and output are commonly used (Non-Patent Document 1). Non-Patent Document 1: Jon G, Kassakian, Principles of power electronics, ADDISON-WESLEY PUBLISHING COMPANY, 1992, pp. 674-687